A youth, affected by what activists say is nerve gas, is treated at a hospital in the Duma neighbourhood of Damascus (Reuters)

The attack that killed over 200 people in the Damascus suburbs could be an accident caused by a riot control agent, and those responsible may be a faction of the Free Syrian Army (FSA) and not the Assad regime, according to a chemical weapons specialist.

Syrian opposition activists claim that rockets with toxic agents were launched towards the suburbs of the Ghouta region at 3am killing more than 570 people, mostly women and children. Horrific footage of people being treated in makeshift hospitals and many children suffocating, having convulsions or laying immobile emerged on social media.

But Gwyn Winfield, editorial director at CBRNe World, a magazine specialising in chemical, biological, radiological, nuclear and explosive weapons, told IBTimes UK that it is difficult to identify an agent by the signs and symptoms alone.

"We can say there is some form of chemical used. But until we take blood samples, samples of the soil and the water in the area we can't say that a chemical agent has been used," he said.

"It can be a riot control agent, like in the recent Egyptian case of the tear gas used in the back of a van that killed 36 prisoners. We have the same symptoms in the children and casualties."

In response to the initial reports, the Syrian government has denied that chemical weapons were used.

Winfield said he finds it "suspicious" that in the week UN inspectors enter the country with the acquiescence of Assad, the Government welcomes them with a chemical weapons missile barrage.

"It is not impossible that some faction in the Free Syrian Army did it to get attention or maybe it was an accident by an inexperienced operator who may have not realised what he was doing."

The security expert said there are numerous cases of riot agent used in a lethal way, for example in confined spaces and not to disperse a crowd.

He added that the idea that the FSA is single and unified is "a myth," because "there are different factions and something like that could be a win/win for them: launch the attack and raise amount of profile of their cause by the UN."

Winfield said that more than 570 victims, if confirmed, are a lot of people in a not densely populated area. "It means the attack targeted a large area and it is not an isolated incident. If there are 570 fatalities it would suggest that evidence of what it is will not be hard to find."

Hamish de Bretton-Gordon, a former commanding officer at the UK's Joint Chemical Biological Radiological Nuclear Regiment, told the BBC's Today programme that the footage was "horrific" and agreed that it would be "very difficult to stage-manage".

The UK Foreign Secretary William Hague said he is "concerned" by reports of a chemical attack.